Due to improved Internet infrastructures and the widespread of communication terminals, such as cellular phone terminals, personal computers, and VoIP (Voice over Internet Protocol) phone sets, the number of Internet users is now exploding. Under such circumstances, security problems such as computer viruses, hacking and spam mails have become apparent, requiring appropriate techniques for communication control.
The Internet has enabled easy access to a vast amount of information. On the other hand, harmful information is proliferating thereon and regulation on its originator does not keep up with the proliferation. To provide an environment where everyone can use the Internet safely and effectively, there is required an appropriate technique for controlling access to harmful contents.
For example, there has been proposed an access control technique in which are prepared databases containing lists of sites to which access is permitted or prohibited, forbidden keywords or useful keywords, so as to control access to external information via the Internet with reference to such databases (see Patent Document 1, for example).    [Patent Document 1] Japanese Patent Application Laid-open No. 2001-282797.